Fix Docker environment and improve reproducibility
Fix Docker environment and improve reproducibility

* Have guild work in the Docker environment
    * For some reason it installs a broken virtualenv==20.0.0b2
    * Set LANG
    * Use python3 and pip3 since python2.7 had reached EOL.
* Increase reproducibility
    * Explicitly use Ubuntu 18
    * No apt-get upgrade
* Follow Docker best practices for Ubuntu base containers

guildai/guild,guildai/guild,guildai/guild,guildai/guild